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We combine the exceptional depth of the Hubble Ultra Deep Field (HUDF) imagesand the deep GRism ACS Program for Extragalactic Science (GRAPES) grismspectroscopy to explore the stellar populations of 34 bulges belonging tolate-type galaxies at z=0.8-1.3. The sample is selected based on the presenceof a noticeable 4000A break in their GRAPES spectra, and by visual inspectionof the HUDF images. The HUDF images are used to measure bulge color and Sersicindex. The narrow extraction of the GRAPES data around the galaxy centerenables us to study the spectrum of the bulges in these late-type galaxies,minimizing the contamination from the disk of the galaxy. We use the lowresolution (R~50) spectral energy distribution (SED) around the 4000A break toestimate redshifts and stellar ages. The SEDs are compared with models ofgalactic chemical evolution to determine the stellar mass, and to characterizethe age distribution. We find that, (1) the average age of late-type bulges inour sample is ~1.3 Gyr with stellar masses in the range log(M)=6.5-10 solar.(2) Late-type bulges are younger than early-type galaxies at similar redshiftsand lack a trend of age with respect to redshift, suggesting a more extendedperiod of star formation. (3) Bulges and inner disks in these late-typegalaxies show similar stellar populations, and (4) late-type bulges are betterfitted by exponential surface brightness profiles. The overall picture emergingfrom the GRAPES data is that, in late-type galaxies at z=1, bulges form throughsecular evolution and disks via an inside-out process.

Stellar Populations of Late-Type Bulges at z=1 in the HUDF